The primary objective of this research is to determine the importance of learning in the acquisition of vocal skills in non-human primates. Specific aims are to document the development of vocal production of the individually distinct and gender-preferred chuck calls of squirrel monkeys and determine the extent of vocal learning in the production of calls in this species. The chuck call in squirrel monkeys is acoustically different in infants and adults, is emitted primarily by adult females during close affiliative interactions, and shows a strong degree of individuality in acoustic structure. This suggests that learning may play an important role in the maturation of this vocal subsystem. Four social groups consisting of 4-5 adult females and their offspring (0-24 months) are the subjects of this study. Subjects are observed and acoustically recorded using a focal animal sampling design and a combination of continuous, interval, and one-zero sampling techniques. Chucks will be acoustically compared and analyzed with respect to the social relationships between infants/juveniles and adult females. Data collection and analysis is currently in progress. Preliminary results suggest that infant/juvenile squirrel monkeys show social preferences for particular genetically-unrelated adult females. Social preference was measured by the amount of time infants/juveniles spend in proximity, contact, and social huddling with different adult females as well as in the amount and direction of activities such as approaches, social exploration, social play, retrievals, and agonisitic interactions. These data suggest that sufficient variation exists between infants in their social preferences to evaluate the role of vocal learning and social influence on chuck call development. Over 500 chuck calls have been digitized for quantitative acoustic analysis. Acoustic comparison will reveal whether chuck calls of infants/juveniles resemble those of their mothers or those of preferred social partner(s). *KEY* Vocal learning, Language development, Evolution of language, Bioacoustic analysis